Selenium or tellurium treated silica

ABSTRACT

A process comprising treating silica with at least one of selenium or tellurium under nonoxidizing conditions at a temperature of at least 370° C. and a subsequent second step wherein the thus-treated silica is contacted with an oxidizing ambient at a temperature within the range of 370°-900° C. The silica can contain a chromium compound in which case the resulting product is suitable as a catalyst for the production of higher melt flow olefin polymers suitable for such applications as injection molding, and the like requiring a relatively narrow molecular weight distribution.

BACKGROUND OF THE INVENTION

This invention relates to the treatment of silica, particularlysilica-supported chromium catalysts suitable for olefin polymerization.

Supported chromium oxide catalysts can be used to prepare olefinpolymers in a hydrocarbon solution to give a product havingcharacteristics from many standpoints. Supported chromium oxidecatalysts can also be used to prepare olefin polymers in a slurry systemwherein the polymer is produced in the form of small particles of solidmaterial suspended in a diluent. This process, frequently referred to asa particle-form process, has the advantage of being less complex.However, certain control operations which are easily carried out in thesolution process are considerably more difficult in the particle-formprocess. For instance, in the solution process, control of the molecularweight can be effected by changing the temperature with lower molecularweight (higher melt flow) being obtained at the higher temperatures.However, in the slurry process, this technique is inherently limitedsince any effort to increase the melt flow to any appreciable extent byincreasing temperature will cause the polymer to go into solution andthus destroy this slurry or particle-form process. Also it is, ofcourse, desirable to achieve high catalyst activity so as to make moreefficient use of a given size reactor.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a catalyst capable ofgiving high melt flow polymer; it is a further object of this inventionto provide a catalyst suitable for use in slurry polymerization systems;it is a further object of this invention to provide an improved methodof activating a chromium-containing catalyst; it is yet a further objectof this invention to provide a catalyst capable of giving high activityin addition to high melt flow and narrow molecular weight distribution;it is still yet a further object of this invention to provide a catalystwhich offers greater latitude in activation temperature; it is still yeta further object of this invention to treat silica to produce a materialparticularly suited for use as a support for a chromium catalystcomponent; and it is still yet a further object of this invention toprovide a catalyst capable of giving polymer suitable for injectionmolding and other applications requiring high melt flow and narrowmolecular weight distribution.

In accordance with this invention, a silica composition is treated at anelevated temperature under nonoxidizing conditions with a treating agentselected from selenium, tellurium, and mixtures thereof and thereaftersubjected to oxidizing conditions.

BRIEF DESCRIPTION OF THE DRAWING

The drawing, forming a part hereof, is a plot of melt index of theresulting polymer as a function of catalyst oxidation temperature.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The silica can be any silica-containing gel as broadly disclosed, forinstance, in Hogan et al, U.S. Pat. No. 2,825,721 (Mar. 4, 1958), thedisclosure of which is hereby incorporated by reference. The silica cancontain, if desired, from about 0.1 to 20 weight percent of materialsother than silica, such as alumina and titania as is known in the art.The silica is preferably a cogel, that is, a silica produced bycoprecipitating silica and a titanium-containing compound. Production ofsuch cogels is described in Dietz, U.S. Pat. No. 3,887,494 (June 3,1975), the disclosure of which is hereby incorporated by reference.Generally, the titanium compound is present in an amount sufficient togive 0.1 to 10, preferably 0.5 to 5 weight percent titanium based on theweight of the final oxidized (activated) catalyst. Cogels can be formed,for instance, by adding a titanium compound to a mineral acid,introducing an alkali metal silicate into the acid containing saidtitanium compound to form a hydrogel, aging the hydrogel for greaterthan one hour, washing the thus aged hydrogel to produce a substantiallyalkali metal free hydrogel, forming a mixture comprising the thus washedhydrogel and a normally liquid oxygen-containing water soluble organiccompound, and separating said organic compound and water from saidmixture to form a xerogel.

When the silica is used as a chromium catalyst base, the final activatedcatalyst contains chromium in an amount generally within the range of0.001 to 10, preferably 0.1 to 5, more preferably about 0.5 to 1 weightpercent based on the weight of the final oxidized (activated) catalyst.The chromium compound can be incorporated as known in the art. Forinstance, a hydrocarbon solution of a material such as tertiary butylchromate can be used to impregnate the xerogel; or an aqueous solutionof a chromium compound such as ammonium chromate, chromium acetate,chromium nitrate or chromium trioxide, preferably chromium trioxide orchromium acetate can be added to the hydrogel before drying; or chromiumcan be coprecipitated along with the silica or the silica and titanium.Anhydrous hydrocarbon solutions of π-bonded organochromium compoundssuch as diarene chromium compounds, for instance dicumene chromium, orbiscyclopentadienyl chromium II can be used. In U.S. Pat. No. 3,976,632(Dec. 4, 1974; U.S. Pat. No. 3,349,067 (Oct. 24, 1967) and U.S. Pat. No.3,709,853 (Jan. 9, 1973), the disclosures of which are herebyincorporated by reference, are disclosures of suitable chromiumcompounds.

Since some of the beneficial effect of the selenium or telluriumtreatment of silica supported chromium catalysts comes from its effecton the silica itself, chromium-free silica can be treated, i.e.,subjected to high temperature contact with selenium or tellurium undernonoxidizing conditions and thereafter subjected to the oxidizing stepafter which the chromium, preferably in the hexavalent state, isintroduced.

The amount of selenium and/or tellurium, calculated as the element,admixed with the silica can range from about 0.005 to about 2 weightpercent, more preferably from 0.05 to 1 weight percent and even morepreferably from 0.05 to 0.2 weight percent based on the weight of thefinal oxidized silica composition. One further surprising feature ofthis invention is the low level selenium or tellurium which iseffective. In catalyst applications, an amount of only one-half or lessof that necessary to convert all of the chromium to the selenide ortelluride can be used.

Elemental selenium or tellurium can be employed. Also, organic compoundsof these elements which decompose to the elemental form under theactivation conditions subsequently used can be employed. Suitablematerials, for example, include selenols, selenides, tellurols, andtellurides which contain hydrocarbyl radicals containing from 1 to about20 carbon atoms. Examples of specific compounds include dimethylselenide, dieicosyl selenide, diphenyl selenide, dicyclohexyl selenide,ethaneselenol, benzenetellurol, di-n-butyl telluride, tetraphenyltelluride, and the like and mixtures. With selenium the elemental formis preferred because it vaporizes at about the ideal temperature for thecatalyst activation. With tellurium a compound is preferred.

When the object is to produce a chromium containing catalyst, thechromium is preferably at least partially, more preferably predominantlyin the hexavalent state on initial contact with the selenium ortellurium. If a chromium compound having a lower valence such aschromium acetate is used, it is preferred to preoxidize the chromiumprior to contact with the selenium or tellurium. Even if CrO₃ is used,this may still be desirable. While applicant does not wish to be boundby theory, it is believed that the selenium or tellurium treatment hasan effect on the chromium, probably making it easier to be reoxidized,especially if it is in the hexavalent state on initial contact with thetreating agent, in addition to the effect it has on the silica itself.

The temperature for contact with oxygen prior to treating with seleniumor tellurium, if oxygen treatment used at all at this point, isgenerally from room temperature up to the treating temperature, althoughit is contact with air at a temperature of 250°-1000° C., preferably700°-925° C., is most advantageous. The time can simply be whatever timeis required to heat the material to treating temperature and generallywill be greater than 5 minutes, preferably 5 minutes to 15 hours, morepreferably 20 minutes to 10 hours, most preferably 40 minutes to 3hours. The heating may be continuous or stopped at any point within theabove recited temperature ranges.

The treatment of the catalyst with the selenium or tellurium is mostconveniently carried out in a fluidized bed and must be carried out atan elevated temperature. This must be done in a nonoxidizing ambient,preferably carbon monoxide or a mixture of carbon monoxide and nitrogen.About 2-100 volume percent CO and 98-0 percent N₂, for instance, can beutilized, other nonoxidizing ambients include inert ambients such ascarbon dioxide, vacuum, helium, and nitrogen, and reducing ambients inaddition to CO such as hydrogen, and materials which decompose into COand/or H₂ at the activation temperatures employed such as hydrocarbons,alcohols, ammonia and carboxylic acids.

The treating agent, if a solid or a high boiling point liquid, can beadmixed with the silica prior to the treating process itself.Alternatively, the treating agent, if a volatile liquid, can be admittedin the form of a gas or vapor to the fluidized bed and convenientlyadmixed with the fluidizing medium, e.g. nitrogen. Fluidizing drycatalyst (chromium on a silica-containing base) with nitrogen removesair or oxygen if air or oxygen has been used to preoxidize the chromiumprior to treating with the selenium or tellurium. If nitrogen has beenused initially, then, of course, the same nitrogen fluidization cancontinue. It is preferred that the contact of the preoxidized catalystwith nitrogen at this elevated temperature before treating be veryshort, preferably less than 30 minutes, more preferably less than 5minutes, most preferably about 1 minute or less, i.e., the minimum timerequired to remove oxygen to prevent self-reduction of the Cr in thecatalyst. Also, if an oxygen-containing gas such as air is used in theinitial heating, it is essential to remove it if organic selenium ortellurium compounds are used since the contact at this temperaturebetween oxygen and the organic material might result in uncontrolledoxidation.

The temperatures employed in treating the silica with selenium ortellurium (either as the element or a compound thereof) is at leastabout 370° C. and generally can range from about 370° to about 1000° C.,preferably from about 700° to 950° C. The times can be relatively short,generally longer than 1 minute, preferably from about 5 minutes to about5 hours.

The oxidation of the treated composition can take place in any oxidizingambient containing oxygen, the preferred ambient, of course, being air.Air-nitrogen mixtures containing 10-100 percent air and 0-90 percentnitrogen can be utilized (gas percentages are all in volume percent). Inaddition, oxidizing ambients such as NO₂, N₂ O, and oxygen-containinghalogen materials such as I₂ O₅ or Cl₂ O can be utilized.

The oxidation temperatures employed with the treated silica can rangefrom the same temperature used for the treating step down to about 50percent or less of the treating temperature. Preferably, the temperatureis at least 50° C. lower in the oxidation step. Actual temperaturespreferably range from about 370°-900° C. After selenium treatment, thetemperature preferably ranges from 400°-700° C., most preferably from425°-650° C., and after tellurium treatment it preferably ranges from400°-600° C.

The oxidation time generally is in excess of 5 minutes and usuallyranges from about 1/2 to 10 hours, preferably from 1 to 4 hours.

If desired, the chromium-containing catalyst of this invention can beactivated in a continuous activator. For instance, catalyst can beintroduced at the top of a compartmentalized vertical activator with thefirst gas to be used in treating the catalyst being introduced at thebottom of the first (upper) compartment and taken off near the topthereof. The second gas is introduced near the bottom of the second(lower) compartment and taken off near the top thereof and if three ormore gases are used, the process can continue in a like manner. In eachcompartment, the catalyst would be fluidized with the treating medium.Alternatively, two or more compartments could be utilized with thegaseous treating medium, if desired, to increase residence time. Anexternal furnace can heat each compartment to the desired temperature.Otherwise, a conventional batch activator can be used.

The catalysts of this invention can be used to polymerize at least onemono-1-olefin containing 2 to 8 carbon atoms per molecule. Thesecatalysts are of particular applicability in producing ethylenehomopolymers and copolymers from mixtures of ethylene and 1 or morecomonomers selected from aliphatic mono-1-olefins containing from 3 to10 carbon atoms and/or conjugated or nonconjugated dienes containing 4to 8 carbon atoms per molecule. Exemplary comonomers include propylene,1-butene, 1-hexene, 1-octene, 1-decene, and other aliphatic 1-olefinsand mixtures thereof and 1,3-butadiene, isoprene, piperylene,2,3-dimethyl-1,3-butadiene, 1,4-pentadiene, 1,7-hexadiene, and otherconjugated or nonconjugated dienes and mixtures thereof. Ethylenecopolymers and terpolymers preferably constitute about 90, preferably 95to 99 mole percent polymerized ethylene units. Ethylene, propylene,1-butene and 1-hexene are especially preferred.

The polymers can be prepared from the activated catalysts of thisinvention by solution polymerization, slurry polymerization, and gasphase polymerization techniques using conventional equipment andcontacting processes using either batchwise or continuouspolymerization. However, the catalysts of this invention areparticularly suitable in slurry polymerizations for the production ofhigh melt index (MI) polymers, i.e., polymers having MI values generallyin the 0.5-30 range and above in the absence of molecular weightmodifiers, such as hydrogen, and with a molecular weight distributionvalue sufficiently narrow to be of commercial interest for applicationssuch as injection molding. The slurry process is carried out in an inertdiluent such as a paraffin, cycloparaffin or aromatic hydrocarbon at atemperature at which the resulting polymer is insoluble. Forpredominantly ethylene polymers, the temperature is about 66°-110° C.The MI goes up and the HLMI/MI ratio goes down at higher reactortemperatures. Thus, for any comparison between the invention runs andcontrol runs to be meaningful, they must be carried out at the samepolymerization temperature.

The catalysts of this invention can be used with conventionalcocatalysts such as triethylaluminum if desired. Also, hydrogen can beused to further increase the MI if desired.

The polymers produced with the catalysts of this invention can beemployed in the production of such articles as fibers, film, sheet,containers, toys, and the like by using conventional plasticsfabrication equipment.

EXAMPLE 1 Control Catalysts and Ethylene Polymerization Therewith

Two types of control catalysts, designated as cogel and MS-Ti wereutilized in particle form ethylene polymerization to provide a basis ofcomparison with the later described invention catalysts.

The cogel referred to hereinafter was a coprecipitated silica-titaniagel comprising about 1.96 weight percent titanium based on the totalweight of the activated catalyst. Sufficient CrO₃ solution was used toimpregnate the wet gel to impart 0.84 weight percent chromium based onthe total weight of the activated catalyst. Following impregnation, thewet composite was dried by means of extraction with liquid ethylacetate.

MS-Ti is derived from a commercially available microspheroidal silica(952 available from Davison Chemical Company), each sample of which wasimpregnated with chromium acetate and titanium triethanolamine complexsufficient to provide the amounts of chromium and titanium shown inTable I.

Each dry catalyst was activated for polymerization in a fluidized bed byemploying a 48 mm O.D. quartz tube charged with about 50 ml (11 g) ofthe catalyst. The activator was raised from about 25° C. to 870° C.using a heat-up rate of 3°-5° C./minute and in the presence offluidizing gas consisting of 5 volume percent CO and 95 volume percentnitrogen amounting to about 42 liters/hour. The gas used corresponds toa superficial linear velocity of about 0.1 ft. (0.3 cm) per second. Thesample was held at 870° C. for four hours, the CO flow was cut off andthe temperature of the sample reduced to the indicated oxidationtemperature in nitrogen. Nitrogen flow was stopped, dry air substituted,and the sample was calcined (oxidized) for two hours at the indicatedtemperature. The activated catalyst was then recovered and stored in adry atmosphere until ready for polymerization testing.

A suitable portion of each catalyst was charged to a stirred stainlessreactor of 2 liter capacity containing 1.5 lbs. (682 g) isobutane as adiluent. The reactor and contents were raised to 107° C. or asspecified, ethylene was admitted to give a reactor pressure of 550 psig(3.79 MPa) and the run was continued until sufficient polymer was formedto provide an estimated productivity value of about 5000 g polymer per gcatalyst. Ethylene was admitted on demand during the run to maintain thepressure.

When the estimated productivity was reached the run was terminated bystopping the ethylene flow, the ethylene remaining in the reactor andisobutane diluent were removed by flashing and the polymer wasrecovered, dried and weighed. The melt index measurements of thepolymers were determined in accordance with ASTM D 1238-65T, condition E(melt index) and condition F (high load melt index).

The catalyst oxidation temperatures employed in activation, analyzedcatalyst properties, catalyst weights, and results obtained are given inTable I. Pore volumes were determined on the catalysts after activation.Titanium values are analyzed for cogel and calculated for the MS-Ticatalysts. The melt index (MI) values listed have been corrected to acommon productivity level of 5000 g polymer per g catalyst to provide acomparative basis for judging the MI results. Pore volumes represent thenumber of cc of isopropanol adsorbed per gram of catalyst according tothe Innes method as described in Analytical Chemistry 28, 332-334(1956).

                                      TABLE I                                     __________________________________________________________________________    Control Catalysts and Ethylene Polymerization Results                         Catalyst                                         Max.                                Sur.                                                                             Pore       Ox.    Reac.    Cal.                                                                              Polymer Polym.                       Run    Area                                                                             Vol.                                                                             Wt. %                                                                             Wt. %                                                                             Temp.                                                                             Wt.                                                                              Temp.                                                                             Polymer                                                                            Prod.                                                                             Corr.                                                                             HLMI                                                                              Rate                         No.                                                                              Type                                                                              m.sup.2 /g                                                                       cc/g                                                                             Cr(VI)                                                                            Ti  °C.                                                                        mg °C.                                                                        Yield, g                                                                           g/g cat                                                                           MI  MI  g/g/hr.                      __________________________________________________________________________    1  cogel                                                                             448                                                                              2.24                                                                             0.25                                                                              1.86                                                                              538 65.9                                                                             107 352  5340                                                                              1.9 51  3100                         2  cogel                                                                             442                                                                              2.27                                                                             .33 1.86                                                                              593 57.3                                                                             107 313  5460                                                                              2.7 50  3770                         3  cogel                                                                             451                                                                              2.25                                                                             .33 1.86                                                                              649 61.7                                                                             107 337  5460                                                                              5.5 41  4860                         4  cogel                                                                             439                                                                              2.24                                                                             .39 1.86                                                                              704 45.0                                                                             107 224  4980                                                                              5.8 42  5070                         5  cogel                                                                             442                                                                              2.29                                                                             .59 1.86                                                                              760 36.0                                                                             107 209  5810                                                                              7.4 42  6170                         6  cogel                                                                             446                                                                              2.26                                                                             .71 1.86                                                                              816 43.0                                                                             107 228  5300                                                                              3.4 46  5580                         7  cogel                                                                             442                                                                              2.27                                                                             .83 1.86                                                                              871 48.4                                                                             107 238  4920                                                                              2.3 56  5580                         8  MS-Ti                                                                             290                                                                              1.52                                                                             .39 1   538 32.3                                                                             107 137  4240                                                                              1.6 65  6130                         9  MS-Ti                                                                             280                                                                              1.43                                                                             .25 3   538 40.6                                                                             104.5                                                                             179  4410                                                                              3.8 63  4290                         10 MS-Ti                                                                             273                                                                              1.31                                                                             .20 5   538 42.0                                                                             104.5                                                                             219  5210                                                                              5.6 68  2860                         __________________________________________________________________________

The control run results are typical for catalysts given the indicatedactivation conditions and employed in particle form ethylenepolymerization. The activation treatment afforded supported chromiumoxide catalysts with CO-N₂ mixtures followed by oxidation in air and theeffect of various temperatures is disclosed in U.S. Pat. No. 4,151,122which issued Apr. 24, 1979 to M. P. McDaniel and M. B. Welch.

EXAMPLE II Invention catalysts and Ethylene Polymerization Therewith

Samples of the dry, nonactivated catalysts described in Example I wereindividually admixed with either elemental selenium or tellurium inpowder form. Each composite was subsequently activated forpolymerization as described in the first example.

The catalyst oxidation temperatures employed in activation, analyzedcatalyst properties, catalyst weights and results obtained are given inTable II. As before, the MI values shown have been corrected to a commonproductivity level of 5000 g polymer per g catalyst.

                                      TABLE II                                    __________________________________________________________________________    Se-- Or Te-- Treated Catalysts and Ethylene Polymerization Results            Catalyst                                           Max.                              Sur.                                                                             Pore   Wt. %  Ox.    Reac.                                                                             Polymer     Prod.                                                                             Polym.                     Run    Area                                                                             Vol.                                                                             Wt.%    Cr Temp.                                                                             Wt.                                                                              Temp.                                                                             Yield                                                                             Corr.                                                                             HLMI                                                                              g/g Rate                       No.                                                                              Type                                                                              m.sup.2 /g                                                                       cc/g                                                                             Ti  Se                                                                              Te                                                                              (VI)                                                                             °C.                                                                        mg °C.                                                                        g   MI  MI  cat.                                                                              g/g/hr.                    __________________________________________________________________________    12 cogel                                                                             437                                                                              2.23                                                                             1.86                                                                              0.1                                                                             0 0.63                                                                             425 60.6                                                                             107 324 9.3 39  5350                                                                              6340                       13 cogel                                                                             440                                                                              2.25                                                                             1.86                                                                              0.1                                                                             0 .78                                                                              480 52.5                                                                             107 284 9.4 44  5410                                                                              6970                       14 cogel                                                                             444                                                                              2.24                                                                             1.86                                                                              0.1                                                                             0 .73                                                                              538 51.9                                                                             107 273 12.3                                                                              36  5260                                                                              5900                       15 cogel                                                                             436                                                                              2.24                                                                             1.86                                                                              0.1                                                                             0 .73                                                                              593 44.1                                                                             107 223 9.5 42  5060                                                                              8160                       16 cogel                                                                             437                                                                              2.26                                                                             1.86                                                                              0.1                                                                             0 .81                                                                              649 34.4                                                                             107 173 9.2 41  5030                                                                              8370                       17 cogel                                                                             446                                                                              2.26                                                                             1.86                                                                              0.1                                                                             0 .81                                                                              704 41.7                                                                             107 215 6.2 46  5160                                                                              6900                       18 cogel                                                                             455                                                                              2.26                                                                             1.86                                                                              0.1                                                                             0 .83                                                                              760 45.4                                                                             107 217 5.5 41  4780                                                                              7530                       19 cogel                                                                             435                                                                              2.29                                                                             1.86                                                                              0.1                                                                             0 .86                                                                              816 47.6                                                                             107 265 5.7 41  5570                                                                              7690                       20 cogel                                                                             438                                                                              2.17                                                                             1.86                                                                              0.1                                                                             0 .87                                                                              871 35.1                                                                             107 193 3.5 44  5500                                                                              10940                      21 cogel                                                                             427                                                                              2.29                                                                             1.86                                                                              0 0.1                                                                             .09                                                                              538 48.6                                                                             107 251 5.0 42  5160                                                                              2720                       22 cogel                                                                             429                                                                              2.28                                                                             1.86                                                                              0 0.1                                                                             .13                                                                              593 62.3                                                                             107 327 5.2 43  5250                                                                              3560                       23 cogel                                                                             427                                                                              2.31                                                                             1.86                                                                              0 0.1                                                                             .23                                                                              649 46.7                                                                             107 242 4.7 46  5180                                                                              4880                       24 MS-Ti                                                                             296                                                                              1.48                                                                             1   0.1                                                                             0 .45                                                                              538 43.3                                                                             104.5                                                                             211 1.0 79  4870                                                                              6650                       25 MS-Ti                                                                             288                                                                              1.41                                                                             3   0.1                                                                             0 .27                                                                              538 44.4                                                                             104.5                                                                             228 3.4 71  5140                                                                              4460                       26 MS-Ti                                                                             283                                                                              1.33                                                                             5   0.1                                                                             0 .25                                                                              538 48.5                                                                             104.5                                                                             219 7.8 57  4520                                                                              4330                       27 MS-Ti                                                                             279                                                                              1.25                                                                             7   0.1                                                                             0 .20                                                                              538 40.3                                                                             104.5                                                                             232 6.6 72  5760                                                                              2680                       __________________________________________________________________________

Inspection of the results presented in Table II shows in runs 12-20 thesuperior behavior of the selenium-treated catalysts compared to therespective control catalysts of runs 1-7 in Table I with respect to meltindex capability or/and polymerization activity. The best controlcatalysts of runs 3-5 produce polyethylenes having MI values rangingfrom 5.5 to 7.4. These catalysts are oxidized at temperatures rangingfrom 649°-760° C. representing a fairly narrow oxidation temperaturerange of 111° C. The best invention catalysts in runs 12-16, incontrast, are oxidized at temperatures ranging from 425°-649° C. andproduce polyethylenes having MI values ranging from 9.2 to 12.3. Theinvention catalysts thus exhibit a much broader oxidation temperaturerange of 224° C. (actually greater than this since temperatures below425° C. can be used) and produce polyethylenes of substantially highermelt index values at higher rates than the control catalysts. Runs18-20, while oxidized at a temperature outside the preferred range foroptimum MI still demonstrate the advantages in activity obtainable inaccordance with the invention. For instance, at 704° C. oxidationtemperature, the MI values are essentially the same, but the activity ofthe invention catalyst is greater (6900 vs 5070). Thus, the preferredtemperature ranges are those which a view of the Figure show give ahigher MI than the control. But the invention is broadly applicable tohigher oxidation temperatures where there is still an advantage inactivity. The invention catalyst of the preceding runs are all based onchromium oxide supported on a silica-titania cogel. In control runs 8-10of Table I and invention runs 24-27 of Table II, the catalysts consistof chromium oxide supported on fast titanated silica. The resultsindicate that a selenium treatment (runs 24-27) provides only marginallyimproved catalysts with respect to polymerization activity and mixedresults with respect to melt index capability. This is believed to bepartially because the chromium was not in the hexavalent state oninitial contact with the selenium, i.e., chromium acetate was usedinstead of CrO₃ and it was not given a preoxidation treatment.

The results obtained with the tellurium-treated catalysts of runs 21-23indicate the melt index capability of such catalysts is superior to thecorresponding control catalysts providing that an oxidation temperatureof no higher than about 600° to perhaps about 625° C. is employed duringthe activation sequence. Thus, the tellurium-treated catalysts appear torespond somewhat differently to oxidation temperatures than theselenium-treated catalysts. Since tellurium has a much higher boilingpoint (1390° C.) than selenium (688° C.) the selenium all vaporizedduring the CO-N₂ treating stage of the activation (871° C.) whilepresumably only a small portion of the tellurium vaporized. It isspeculated that the vaporized selenium or tellurium influences thereaction between the catalyst and the CO in some fashion. The reasoningfor this is that the amount of selenium or tellurium employed in theexamples is about 1/2 or less that quantity required to convert all thechromium in the catalyst to the selenide or telluride compound.

While this invention has been described in detail for the purpose ofillustration it is not to be construed as limited thereby but isintended to cover all changes and modifications within the spirit andscope thereof.

I claim:
 1. A process comprising a first step wherein silica iscontacted with a treating agent selected from at least one of seleniumor tellurium under nonoxidizing conditions at a temperature of at least370° C. and a subsequent second step wherein it is contacted with anoxidizing ambient at a temperature within the range of 370°-900° C.
 2. Amethod according to claim 1 for producing a catalyst wherein said silicacontains a chromium compound and is heated in the presence of oxygenprior to said treatment with said selenium or tellurium, said seleniumor tellurium being in elemental form and being used in an amount withinthe range of 0.05 to 1 weight percent of the element based on the weightof the oxidized catalyst and wherein said oxidizing ambient comprisesoxygen.
 3. A method according to claim 2 wherein after said heating inthe presence of oxygen prior to said treatment, a nitrogen ambient isintroduced to flush out the air prior to said introduction of saidselenium or tellurium.
 4. A method according to claim 3 wherein saidnitrogen flush is carried out for less than 30 minutes.
 5. A methodaccording to claim 2 wherein said treatment is with elemental seleniumunder reducing conditions at a temperature within the range of 700°-950°C. and said oxidation is carried out at a temperature within the rangeof 400°-700° C., which oxidation temperature is also at least 50° C.lower than said temperature for treatment with said selenium.
 6. Amethod according to claim 2 wherein said treatment is with telluriumunder reducing conditions at a temperature within the range of 700°-950°C. and said oxidation is carried out at a temperature within the rangeof 400°-600° C., which oxidation temperature is also at least 50° C.lower than said temperature for treatment with said tellurium.
 7. Amethod according to claim 2 wherein said contact with said selenium ortellurium is for a time within the range of 5 minutes to 5 hours andsaid oxidation step is carried out for a time within the range of 1 to 4hours, said selenium or tellurium being used in an amount of one-half orless of that necessary to convert all of the chromium to the selenide ortelluride.
 8. A method according to claim 1 for producing a catalystwherein said silica contains a chromium compound in an amount sufficientto give 0.1 to 5 weight percent chromium in the final activated catalystand wherein said chromium is at least partially in the hexavalent stateon initial contact with said selenium or tellurium.
 9. A methodaccording to claim 2 wherein said silica is a coprecipitated silicatitanium composition containing a titanium compound in sufficient amountto give 0.5 to 5 weight percent titanium in the final activatedcatalyst.
 10. A method according to claim 9 wherein said chromium isincorporated into said catalyst as an aqueous solution of CrO₃ at thehydrogel stage.
 11. A method according to claim 10 wherein said chromiumis incorporated into said silica in the form of chromium acetate and theresulting silica-chromium composition is oxidized to convert at least aportion of said chromium to the hexavalent state prior to said firstcontact with said selenium or tellurium.
 12. A method according to claim1 for producing a catalyst wherein said silica is coprecipitated with atitanium compound which titanium compound is present in an amountsufficient to give 0.5 to 5 weight percent titanium in the finalactivated catalyst, said chromium is added as an aqueous solution of achromium compound to said silica at the hydrogel stage in an amountsufficient to give 0.1 to 5 weight percent chromium in the finalactivated catalyst, said chromium being at least partially in thehexavalent state on initial contact with said treating agent, saidtreating agent being elemental selenium, said selenium being used in anamount within the range of 0.05 to 0.2 weight percent selenium based onthe final activated catalyst, said treatment with said selenium beingcarried out under reducing conditions at a temperature within the rangeof 700°-950° C. and said contact with said oxidizing ambient beingcarried out in air at a temperature within the range of 400°-700° C. 13.A method according to claim 12 wherein said treatment with said seleniumis carried out in the presence of CO.
 14. A catalyst produced by themethod of claim
 13. 15. A treated silica composition produced by themethod of claim
 1. 16. A catalyst produced by the method of claim 2.